Complementary scheduling of hydro-wind-PV-storage systems: Addressing flexibility demand for high-penetration variable renewable energy integration

The integrated operation of hydro-wind-photovoltaic (PV)-storage hybrid energy systems significantly addresses the consumption challenge of variable renewable energy (VRE). However, increasing VRE penetration exacerbates flexibility shortages, undermining system reliability and economic efficiency. Therefore, this study proposes a flexibility-based short-term scheduling method that balances operational reliability with revenue optimization. Firstly, the uncertainty of VRE output is calculated using the Copula function, which helps quantify the demand for flexibility based on the relationship between this uncertainty and transmission capacity. Secondly, the flexibility supply from conventional and pumped-storage hydropower stations is assessed, and a “day-ahead to real-time” scheduling framework aligns flexibility demand with supply, guiding operational decisions. Finally, a case study in China is conducted and demonstrates the primary findings as follows: (1) Flexibility demand patterns are influenced by VRE output levels and system transmission capacity. During VRE surplus periods, upward flexibility demand shows a bimodal pattern with peaks during PV ramp-up phases. Conversely, during VRE shortages, it displays a unimodal pattern peaking at PV output maxima. (2) Incorporating flexibility improves system reliability by 3.44 % and actual revenue by 1.5 %; while flexibility constraints lead to more conservative scheduling and reduced planned revenues, they enhance reliability, decrease unmet generation penalties, and ultimately increase actual economic returns. (3) Each percent load-loss risk reduction can yield 56 million CNY in revenue. Higher confidence increases the demand for flexibility, reducing total generation and planned revenues. However, this also mitigates risk-related losses, resulting in higher realized revenues.